Molecules at the Quantum-Classical Nanoparticle Interface: Giant Mn70 Single-Molecule Magnets of ∼4 nm Diameter

Two Mn70 torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC2H4OH of [Mn12O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H (1) in the presence of NBu(n)4MnO4 and an excess of MeCO2H. The reaction in EtOH afforded [Mn70O60(O2CMe)70(OEt)20(EtOH)16(H2O)22] (2), whereas the reaction in ClC2H4OH gave [Mn70O60(O2CMe)70(OC2H4Cl)20(ClC2H4OH)18(H2O)22] (3). The complexes are nearly isostructural, each possessing a Mn70 torus structure consisting of alternating near-linear [Mn3(μ3-O)4] and cubic [Mn4(μ3-O)2(μ3-OR)2] (R = OEt, 2; R = OC2H4Cl, 3) subunits, linked together via syn,syn-μ-bridging MeCO2(-) and μ3-bridging O(2-) groups. 2 and 3 have an overall diameter of ∼4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8-10 K range and a 3.5 G ac field with oscillation frequencies in the 5-1500 Hz range, revealed frequency-dependent out-of-phase signals below ∼2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming 2 and 3 to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below ∼0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (Ueff) of 23 and 18 K for 2 and 3, respectively. Because the Mn70 molecule is close to the classical limit, it was also studied using a method based on the Néel-Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum-classical nanoparticle interface.